Effect of the number of thiophene rings in polymers with 2,1,3-benzooxadiazole core on the photovoltaic properties

A series of polymers, poly{5,6-bis(decyloxy)-4-(thiophen-2-yl)benzo[c][1,2,5]oxadiazole} (1T-BO20), poly{4-(2,2′-bithiophen-5-yl)-5,6-bis(decyloxy)benzo[c][1,2,5]oxadiazole} (2T-BO20), poly{4-(2,2′-bithiophen-5-yl)-5,6-bis(decyloxy)-7-(thiophen-2-yl)benzo[c][1,2,5]oxadiazole} (3T-BO20) containing 2,1,3-benzooxadiazole derivative and different thiophene rings are synthesized. Effect of the number of thiophene rings on the optical, electrochemical and photovoltaic properties of the polymers is investigated. The maximum absorption wavelength and the optical band gap of the polymers are almost the same, indicating the polymers exhibit similar intramolecular charge transfer effect. The HOMO levels are in the order of 1T-BO20 (−5.60 eV) < 2T-BO20 (−5.45 eV) < 3T-BO20 (−5.36 eV), revealing that the HOMO level of the polymers are dependent of number of thiophene ring in the back bone. Under the illumination of AM 1.5G, 100 mW/cm2, the power conversion efficiency (PCE) of PSCs based on these polymers increases in the order of 1T-BO20 (1.66%), 2T-BO20 (1.71%) and 3T-BO20 (1.92%). Besides, we find that the efficiency of PSCs showed very different responses by the addition of DIO as a processing additive. The devices based on 1T-BO20 and 2T-BO20 with DIO exhibit an enhancement of PCE from 1.66% to 3.65% and from 1.71% to 2.40%, respectively, whereas PCE of the device based on 3T-BO20 with DIO decreased from 1.92% to 1.76%.